The present invention generally relates to an output control apparatus of a laser unit and an adjusting method thereof, and more particularly to an output control apparatus of a laser unit and an adjusting method thereof which are used for a laser printer and the like.
A light source unit used in a laser printer is disclosed, for example, in Japanese Patent Laid Open Publication No. 61-174794. The light source is a so-called semiconductor laser unit. In this semiconductor laser unit, a semiconductor laser element is mounted together with a collimator lens in a housing. A laser beam having a divergent bundle of rays which is emitted from the semiconductor laser element is converted into a laser beam having a parallel bundle of rays by the collimator lens. Then the laser beam passing through the collimator lens is emitted from the housing of the semiconductor laser unit. The laser beam from the semiconductor laser unit is supplied to a scanner and a focusing system so as to scan a photosensitive drum. Thus, an image is formed on the photosensitive drum in accordance with an electrostatic recording procedure.
An example of a structure of the semiconductor laser unit is shown in FIG. 1. Referring to FIG. 1, a semiconductor laser element 1, a monitor photodiode (not shown) and a collimator lens 4 are mounted in a housing which is formed of a metal base plate 2 and an insulator 3. The monitor photodiode is, for example, formed of a pin photodiode. A print circuit board 6 (PCB) is fixed on a rear end surface of the housing. The semiconductor laser element 1, the monitor photodiode, a monitor signal amplifier 8 and other electric components are electrically connected to the print circuit board 6. An aperture 5 for shaping a spot of a laser beam is formed on the front end surface of the housing. In the semiconductor laser unit 7 having the above structure, the semiconductor laser element 1 emits a first beam which travels toward the front end surface of the housing and a second beam which travels toward the monitor photodiode. The first beam is referred to as a front beam (FB) and the second beam is referred to as a back beam (BB).
The semiconductor laser unit is detachably mounted in the laser printer. In a case where the semiconductor laser unit is used in the laser printer, emission power which corresponds to the amount of light emitted from the semiconductor laser unit 7 has to be controlled at a standard power level which is required for exposing the photosensitive drum in the laser printer. Thus, a feed back loop control circuit shown in FIG. 2 controls the emission power of the semiconductor laser unit 7 at the standard power level.
Referring to FIG. 2, the back beam (BB) emitted from the semiconductor laser element 1 is incident on the monitor photodiode 10. The monitor photodiode 10 outputs a monitor current corresponding to the amount of incident light. The monitor signal amplifier 8 converts the monitor current supplied from the monitor photodiode 10 into a monitor voltage V.sub.m. The monitor voltage V.sub.m and a reference voltage V.sub.mo output from a reference voltage generator 9 are supplied to a comparator 14. The comparator 14 outputs a control signal corresponding to a difference between the monitor voltage V.sup.m and the reference voltage V.sup.mo. An LD driver 15 controls a driving current of the semiconductor laser element 1 based on the control signal supplied from the comparator 14 so that the monitor voltage V.sub.m becomes equal to the reference voltage V.sup.mo.
In the above controlling of the emission power of the semiconductor laser unit 7, there is the following problem.
A divergent angle of the laser beam emitted from the semiconductor laser element 1, an arrangement position and sensitivity of the monitor photodiode 10, a transmittance of the collimator lens 4 and the like respectively varies among a plurality of semiconductor laser units. As a result, a relationship between the monitor voltage V.sup.m and the emission power of the semiconductor laser element 1 varies among the plurality of semiconductor laser units. In this case, even if the semiconductor laser element 1 in each semiconductor laser unit is driven based on a constant driving signal, the amount of light projected onto the surface of the photosensitive drum varies among a plurality of laser printers each having the semiconductor laser unit.
Thus, in the conventional semiconductor laser unit, a gain of the monitor signal amplifier 8 is adjusted in accordance with the following procedure.
The emission power regarding the front beam (FB) emitted from the semiconductor laser element 1 is measured by a power meter 13 on the surface of the photosensitive drum. The driving current output from the LD driving circuit 15 is adjusted in accordance with the emission power measured by the power meter 13. Then, in a case where the emission power is equal to the standard power level, the gain of the monitor signal amplifier 8 is controlled so that the monitor voltage V.sup.m is equal to the reference voltage V.sup.mo.
The monitor signal amplifier 8 has, for example, an operational amplifier 81 and a feed back circuit including a variable resistor 82, shown in FIG. 3. For example, a type of resistor in which an adjusting shaft has to be rotated a large number of times to vary the resistance thereof is used as the variable resistor 82. The variable resistor 82 can also be formed of a first resistor which roughly controls the resistance thereof and a second resistor which finely controls the resistance thereof.
However, in the conventional adjusting of the gain of the monitor signal amplifier, the adjusting shaft of the variable resistor has to be rotated. Thus, after the adjustment is completed, the adjusting shaft has to be locked so as to prevent the adjusting shaft from being rotated by vibrations. In this case, it is difficult to automatically adjust the resistance. In addition, in a case where two variable resistors (the above first and second variable resistors) have to be adjusted, the cost of manufacturing the semiconductor laser unit increases, and the time required for adjusting the emission power of the semiconductor laser unit also increases.